This invention relates to a microwave oscillator which is particularly suitable for generating microwave energy at millimeter wavelengths, for example at frequencies above 60 GHz.
More specifically, the invention relates to a microwave oscillator comprising a waveguide having a cut-off frequency, the oscillator further comprising means, including a negative-resistance oscillator device, for generating microwave energy in the waveguide, wherein the generating means are operable to generate microwave energy simultaneously at a fundamental frequency which is below the cut-off frequency of the waveguide and at a harmonic frequency which is an integral multiple of the fundamental frequency and which is above the cut-off frequency, the generating means further comprising first means which in operation couple the oscillator device to the waveguide at the harmonic frequency and on which the values of said fundamental frequency and the harmonic frequency depend.
A typical oscillator of this type using a Gunn diode is described in the article "Oscillators Lock and Tune at W Band" by M. Crandell and F. J. Bernues, Microwave Systems News, December 1980, pages 54-60. As indicated in the article, the resonant disc (also called "resonant cap") arrangement used in the described oscillator is the preferred approach for determining the resonant properties of the millimeter wave circuit. It has been established (see, for example, "Operating Modes of Millimeter Wave Transferred Electron Oscillators" by I. G. Eddison and D. M. Brookbanks, Electronics Letters, Vol. 17, No. 3 (Feb. 5, 1981), pages 112-113, and "Wideband Waveguide System Identifies GaAs Oscillator Harmonics at 94 GHz" by W. H. Haydl, Microwave Systems News, February 1982, pages 99-103) that the negative-resistance oscillator device in such an oscillator generates microwave energy both at a fundamental frequency and at least at a second harmonic thereof. However, since the cut-off frequency of the waveguide in which the oscillator device is disposed is above the fundamental frequency, only the harmonic frequency energy can propagate along the waveguide. The disc or cap is thought to function as a radial line transformer.
For electronically tuning such an oscillator, a varactor diode cannot suitably be used in the manner that is conventional with ordinary oscillators, i.e. spaced along the waveguide from the oscillator device. On the one hand, because the basic oscillation phenomenon occurs at the fundamental frequency, coupling the varactor diode to the oscillator device only at the harmonic frequency would enable it to have very little effect on the oscillating frequencies. On the other hand, placing the varactor diode very close to the oscillator device so that they might be coupled by the evanescent mode present at the fundamental frequency would result in the varactor diode presenting a low impedance across the oscillator device and hence considerably reducing the output power. It is generally not possible to reach an acceptable compromise between a useful width of electronic tuning range and an insignificant reduction in output power.
As mentioned in the article by Crandell and Bernues, the preferred method for electronically tuning such an oscillator is to vary the bias voltage applied to the Gunn diode. However, this has the disadvantages that because of the high tuning sensitivity (e.g. 2 GHz/volt), the bias voltage must be very well regulated to minimize additional FM-noise and must be accurately controlled to achieve the desired frequency variation. Further, the power output varies significantly with the bias voltage and this method can be used to vary the frequency only at a fairly low rate.
To improve the FM-noise performance of the oscillator, the article proposes the use of a phase-locked loop; this requires the use of a considerable amount of additional equipment.
An alternative approach is described in the paper "A Wideband, Backshort-Tunable Second Harmonic W-Band Gunn-Oscillator" by H. Barth, 1981 IEEE MTT-S, International Microwave Symposium Digest, pages 334-337. This paper describes oscillators in which one or more Gunn diodes are disposed in a central portion of a rectangular waveguide, the cut-off frequency of which is below the fundamental frequency of oscillation. One end of this central portion of waveguide is connected to an output rectangular waveguide, the cut-off frequency of which is between the fundamental and second harmonic frequencies of oscillation, so that only harmonic frequency energy is coupled out; the other end of the central waveguide portion containing the diode(s) is connected to a circular waveguide which has the same cut-off frequency (below the fundamental frequency) and which contains two coaxial movable short-circuits, one within the other. The inner short-circuit is operative at the second harmonic frequency, but not the fundamental frequency, and is used in the usual manner to optimize the power output at the second harmonic frequency; the outer short-circuit can be used for mechanically varying the fundamental (and hence the second harmonic) frequency. With a single Gunn diode an electronic tuning range of 300 MHz (at a second harmonic frequency of about 90 GHz) could be obtained by varying the Gunn diode bias voltage; this range is rather small. A configuration which has a wider electronic tuning range comprises two Gunn diodes and a varactor diode spaced along the central rectangular-waveguide portion. With this configuration a tuning range of 1.5 GHz was achieved for a maximum output power variation of 3 dB. This is a rather large variation, and moreover this configuration is mechanically quite complex. Furthermore, it is desirable to improve the noise performance of the oscillator.